1. Field of the Invention
The present invention relates to an endoscope system that scans illumination light over a target to be observed, such as tissue. In particular, it relates to adjusting the brightness of an observed image.
2. Description of the Related Art
An endoscope system with scanning functionality is equipped with a scanning fiber such as a single mode type fiber, which is provided in an endoscope. As described in Seibel et al. (U.S. Pat. No. 6,294,775), the tip portion of the scanning fiber is held by an actuator such as a piezoelectric device, which vibrates the tip portion spirally by modulating and amplifying the fiber's vibration. Consequently, illumination light passing trough the scanning fiber is spirally scanned over an observation area.
Light reflected off the observation area is entered into an image fiber, and is transmitted to a processor via the image fiber. The transmitted light is transformed to image-pixel signals by photosensors, and one frame's worth of image is generated successively. The spiral scanning is periodically carried out on the basis of a predetermined time-interval (frame rate), and one frame's worth of image-pixel signals are read from the photosensors in accordance with the frame rate.
Usually, the amount of illumination light for one frame interval is nearly uniform over an entire observation area. However, if the depth from the scope tip portion to an observation area is not uniform, a part of the object image may become extremely bright and/or dark. For example, when capturing an inner surface of an organ under condition where the scope tip portion faces an axis of the organ, the center portion of the image becomes too dark, whereas the surrounding portion of the image becomes too bright so that a so-called halation occurs in the surrounding portion. On the other hand, when carrying out image processing for correcting the brightness of an observed image, image quality degrades.